High purity nobium suits use in superconducting accelerators

Superconducting accelerators are used to accelerate charged particles (such as electrons, positrons, protons and ionised atoms). Niobium (Nb), which becomes superconductive at the highest temperature (9.25K) among pure metals, is used as material for accelerating cavities. Niobium material for accelerating cavities must have a Residual Resistance Ratio (RRR) that exceeds 250. The RRR is an index of material purity, which is calculated as a ratio of electrical resistivity at room temperature to electrical resistivity above the superconductivity transition temperature (Tc) - the larger the RRR of a material, the higher its purity.

To increase the purity of niobium, ULVAC optimised multiple conditions, including the selection of raw materials, the degree of vacuum, and melting speed by using a 600kW EB melting furnace, thereby successfully producing niobium ingots with an RRR exceeding 250. ULVAC commissioned the High Energy Accelerator Research Organization (KEK) to manufacture a single-cell accelerating cavity by using plates produced from these ingots and to conduct an electric field performance test. Consequently, the company achieved a maximum accelerating electric field gradient of 41MV/m (performance required for ILCs: at least 35 MV/m).

At present, accelerating cavities are manufactured by press-molding niobium plates into cup-shaped parts and welding the parts by electron beams. However, there is concern over a decrease in yield resulting from welding defects, as well as over a decline in acceleration performance caused by welding beads inside cavities. To resolve these issues, ULVAC is conducting joint research and development with KEK on producing accelerating cavities from seamless niobium tubes.

Manufacturing seamless niobium tubes and then bulge-molding the tubes (hydraulic molding: a molding method that places tube-shaped material in a metal mold within a sealed device and imposes high pressure on liquid injected inside the tube to bulge the material into the shape of the mold) will significantly reduce the need for welding, thereby increasing yields as well as avoiding a decline in acceleration performance. By applying this technology, ULVAC will continue making proposals to meet customer needs with niobium material for superconducting accelerators.

The material will be available from July 2015.